Auto merge of #52830 - matthewjasper:bootstrap-prep, r=matthewjasper

[NLL] Fix some things for bootstrap

Some changes that are required when bootstrapping rustc with NLL enabled.

* Remove a bunch of unused `mut`s that aren't needed, but the existing lint doesn't catch.
* Rewrite a function call to satisfy NLL borrowck. Note that the borrow is two-phase, but gets activated immediately by an unsizing coercion.

cc #51823

src/librustdoc/clean/mod.rs

@@ -1172,7 +1172,7 @@ fn resolve(cx: &DocContext, path_str: &str, is_val: bool) -> Result<(Def, Option
 
         // Try looking for methods and associated items
         let mut split = path_str.rsplitn(2, "::");
-        let mut item_name = if let Some(first) = split.next() {
+        let item_name = if let Some(first) = split.next() {
             first
         } else {
             return Err(())